1. Technical field
The disclosure relates to magnetic shift register with pinning structure.
2. Related art
Memory device for storing binary data has wide application. The capacity of memory device is required more and more to adapt various applications in electronic apparatus. In addition, the operation speed is also required be much faster. Recently, the memory device designed as a magnetic shift register has been proposed.
The operation mechanism for the magnetic shift register is based on the movement of magnetic domain wall in a memory track, which have a plurality of magnetic domains. Each magnetic domain serves as a memory cell to store one binary data. The magnetic materials are generally classified into in-plane magnetic anisotropy (IMA) material and perpendicular magnetic anisotropy (PMA) material. FIG. 1 is a drawing, schematically illustrating a conventional magnetic shift register with IMA material. In FIG. 1, an IMA memory track 100 has many magnetic domains 102. In each magnetic domain 102, the magnetization directions 106 thereof as indicated by arrows are lying in the plane. The magnetization direction 106 in each magnetic domain 102 is used to decide the binary data stored in the magnetic domain 102 as “0” or “1”. When the magnetization directions 106 for the two adjacent magnetic domains 102 are different, a domain wall 108 is then created therebetween, as indicated by solid line. When the magnetization directions 106 for the two adjacent magnetic domains 102 are same, no domain wall is created, as indicated by dashed line. A reading device 110, such as magnetic tunnelling junction (MTJ) device, can be implemented adjacent to the memory track 100 to sense the magnetoresistance induced from the passing magnetic domain 102, so as to read the binary data. The write device 112 is also implemented adjacent to the memory track 100 to change the magnetization direction 106 as intended. In order to shift the magnetic domains 102 to pass the write device 112 or the reading device 110, a driving electron current is applied to move the domain walls 108, forward or backward.
In order to have the stable location of domain wall 108, a pinning structure 104, such as a notch structure is formed to stabilize the domain wall 108.
For the PMA material, the magnetization directions 106 in the magnetic domains are perpendicular to the thin film. With a magnetic anisotropic constant (Ku), the magnetic domains of PMA Materials are quite narrow, so that the memory density can be much improved due to the reduced width of the domain wall. FIG. 2 is a drawing, schematically illustrating a conventional magnetic shift register with PMA material. In FIG. 2, the memory track 100 is made by PMA material. In this structure, the magnetization direction 106 in magnetic domain 102 is perpendicular to the plane of the memory track 100. The operation is similar to the magnetic shift register with IMA material as described in FIG. 1.
The magnetic shift register with PMA material may have very narrow domain wall. It may even be less than 10 nm. However, the semiconductor fabrication process based on lithographic and etching processes has limitation of line resolution and therefore is difficult to reduce device size. How to improve the memory density based on the available photolithographic process is still developed.